This invention generally relates to remote monitoring systems and is particularly concerned with a system for electro-magnetically monitoring the condition of a vehicle tire having a plurality of possible states.
It is often times desirable to obtain information about the physical condition of a device disposed at a location remote from the monitoring location. In some environments, an electrical interconnection can be made between the monitoring apparatus and the device through wires and the like. Where such actual physical interconnection is not possible, a so-called "radio link" can be established with the remote device being interrogated by a radio frequency signal transmitted from the monitoring location with the remote device retransmitting a signal to a receiver location, such retransmitted signal containing information indicative of the condition of such device. Typically, the transponding apparatus disposed at the remote device which is being monitored requires an internal source of electrical power and/or requires careful tuning so as to provide stable operation.
In many adverse environments, it is simply not possible to provide an internal source of power for the transponding apparatus, nor is it practical to require such a transponding apparatus to possess critical or sensitive tuning characteristics or the like. One particular adverse environment is that in which the transponding apparatus is disposed on a moving vehicle tire so as to provide reliable information relative to the air pressure within such tire.
A monitoring system is disclosed in the above identified parent patent application, which serves in a reliable fashion to monitor the condition of a remote device, such as a tire for a vehicle, such tire having a plurality of possible states, such as an air pressure which is "too high" or "too low" in accordance with pre-established criteria. In its basic form, the system disclosed in the parent patent application incorporates a transmitter means which serves to transmit an interrogation signal which is composed of a carrier wave modulated at a predetermined modulating frequency. The transmitter means is disposed at a location in close proximity to such tire such as within the vehicle wheel well. A transponder means is further provided preferably at the remote device such as attached to the valve stem of the vehicle tire, such transponder means being operable in response to the state of the device being monitored for passively detecting the modulating frequency of the interrogation signal and retransmitting an information signal at the modulating frequency. The receiver means is also provided preferably in the immediate vicinity of the transmitter which is responsive to the retransmitted information signal from the transponder means for providing an output indicative of the state of the device being monitored, i.e. indicative of the air pressure within the vehicle tire.
The transponder means is constructed of a coil, a non-linear electrical element disposed in a closed series circuit loop and a switch disposed in the loop and actuable to close same in response to the state of the device being monitored. The non-linear circuit element develops a voltage thereacross at the carrier frequency so that a current at the modulating frequency flows through the coil to generate an electro-magnetic field, the field defining the information signal. These three elements are all housed in a compact container and may, for example, be attached to the valve stem of the tire.
Although the vehicle tire pressure monitoring system disclosed in the parent patent application works well, the signal coupling the transmitter, transponder and receiver will be of varying strength due to the variability in the separation distance between the transponder's coil and the transmitter and receiver. This may result in a non-uniform display of the tire pressure by the monitoring system when the vehicle is in motion.